being involved. there's so much we don't know about their motive and operational capabilities, about their planning, how long they had been planning this. we do know a possible suspect, authorities according to john king's sources, believe with some certainty that it is a suspect number two, the 19-year-old dzhokhar, who they have been looking for since last night, since these photos were released, cornered in a boat. can you explain, jeff, flash-bang grenades. we talk about this, seen them in movies. how do they work and why would they be thrown now, if the operator's not moving in? >> well, first of all, the way they work, it's a device that doesn't really have any shrapnel. it has some powder that's designed to burn brilliantly and quickly. pull the pin just like a regular grenade, you throw it and it's got about a two-second delay, then you can adjust the delay before it goes off. it's designed to achieve surprise if you have to enter a room right behind it where there might be armed opposition. but why would they use it now?